ES2834398T3 - Lithium ion secondary battery - Google Patents

Abstract

A lithium ion secondary battery comprising a cathode, an anode, a separator, and an electrolyte, wherein the anode comprises a negative current collector and a negative material layer, and the negative material layer comprises graphite having a grade graphitization of 94% to 98% and an average D50 particle size of 6 µm to 18 µm as negative active material.

Description

DESCRIPTION

Lithium ion secondary battery

TECHNICAL FIELD

[0001] The present application relates to the field of batteries, and more particularly, to a lithium ion secondary battery.

BACKGROUND

[0002] With the increasing popularity of electric vehicles, the requirements of the battery are more stringent, such as the battery has to be both small and light, and also has to have large capacity, long cycle and stable performance. To this end, technicians have made a variety of efforts from the cathode and anode, the battery electrolytes and so on.

[0003] For example, with respect to the positive active material for a battery, NCM has a higher capacity and density compared to lithium ion phosphate (LFP). Therefore, a cell using NCM has a higher energy density. However, unlike LFPs whose volume will shrink when loaded, NCM's volume will expand when loaded; The expansion force will damage the interface between the anode and the cathode and lead to battery failure. Therefore, despite the higher energy density, the life cycle of the ternary battery is often worse than that of the lithium-ion phosphate battery.

[0004] Document JP2015164143 discloses a negative material for a non-aqueous electrolyte secondary battery, wherein the negative material comprises at least: a graphite particle (A) having a surface separation (d002) in a 002 plane of 3 .37 Á or less and a mean circularity of 0.9 or more in a wide angle X-ray diffraction method; and a carbon material (B) having a surface separation (d002) in a 002 plane of 3.37 A or less in a large angle X-ray diffraction method. Said mixed carbon material used as a negative electrode of A non-aqueous secondary battery has the characteristics of a small irreversible capacity.

[0005] Document EP3032620A1 refers to a negative electrode material for a lithium ion battery, made of a composite material comprising silicon-containing particles, artificial graphite particles and a carbon coating layer, wherein the particles Silicon-containing particles are silicon particles that have a layer of SiOx (0 <x <2) on the surface of a particle, have an oxygen content ratio of 1% by mass or more and 18% by mass or less, and they mainly comprise particles having a primary particle diameter of 200 nm or less; and the artificial graphite particles have a scale-like shape. Using the negative electrode material, a lithium ion battery can be produced which has high capacitance and excellent charge-discharge cycle characteristics.

[0006] Document KR20140098424 describes a positive active material for a secondary battery with improved low temperature properties, wherein the positive active material comprises a bimodal lithium nickel-manganese cobalt oxide consisting of small particles and large particles, having different mean particle sizes, and a monomodal iron, lithium and olivine phosphate, where the difference in the content of Ni, Mn and Co in the small particles and the corresponding content of Ni, Mn and Co in the large particles is 2% in moles or less, or the Ni content in the small particles is 1-20% by mole more than the Ni content in the large particles.

[0007] Therefore, it remains a great challenge to match positive right active material with right negative active material in order to improve battery performance.

[0008] In view of the above, it is necessary to provide a battery with good performance.

SUMMARY

[0009] An object of the present application is to provide a lithium ion secondary battery that has comprehensive and balanced performance.

[0010] A further object of the present application is to provide a lithium ion secondary battery capable of providing both long cycle life and high energy density without sacrificing the energy density of the batteries.

[0011] The inventors have experimented with a large number of experiments to surprisingly find that a particular type of positive active material and negative active material for the battery can be combined to improve the cycle life and energy density of the secondary ion battery. of lithium at the same time.

In particular, the present application provides a lithium ion secondary battery comprising a cathode, an anode, a separator and an electrolyte, according to independent claim 1, wherein the anode comprises a negative current collector and a negative material layer, wherein the negative material layer comprises graphite having a degree of graphitization of 94% to 98% and a D50 mean particle size of 6 pm to 18 pm as a negative active material.

Compared to the prior art, the lithium ion secondary battery provided by the present application can have both long cycle life and high energy density through the use of a specific positive and negative active material.

[0014] The present application also relates to a method for producing the lithium-ion secondary battery, which is not part of the present invention, comprising:

1) Prepare a cathode by using a positive active material with the formula LixNiaCobMcO2, where M is at least one selected from the group consisting of Mn and Al, 0.95 <x <1.2, 0 <a <1.0 <b <1,0 <c <1 and to bc = 1;

2) preparing an anode using graphite having a degree of graphitization of 94% to 98% and a D50 average particle size of 6 pm to 18 pm as negative active material; Y

3) Assemble the cathode prepared in step 1) and the anode prepared in step 2) into a battery.

DETAILED DESCRIPTION

[0015] The present application will be described in more detail with reference to the accompanying embodiments and drawings in order to clarify the objects, technical solutions and advantageous technical effects of the present application. It should be understood, however, that the application embodiments are merely for the purpose of explaining the application and are not intended to limit the application, and that the application embodiments are not limited to the embodiments given in the specification. Experimental conditions not specified in the examples are given under conventional conditions, or under conditions recommended by the material supplier.

The present application provides a lithium ion secondary battery comprising a cathode, an anode, a separator and an electrolyte,

wherein the cathode comprises a positive current collector and a layer of positive material, wherein the positive material layer comprises a positive active material with the formula LixNiaCobMeO2, M is at least one selected from the group consisting of Mn and Al, 0.95 < x <1.2, 0 <a <1.0 <b <1.0 <c <1 and bc = 1;

wherein the anode comprises a negative current collector and a negative material layer, wherein the negative material layer comprises graphite having a degree of graphitization of 94% to 98% and a D50 of the average particle size of 6 pm to 18 pm as negative active material.

[0017] The inventors believe that graphite having a degree of graphitization of 94% to 98% and an average particle diameter of D50 from 6 pm to 18 pm can form a high elastic structure within the material, and has an elasticity Greater than conventional graphite material. When charged, the positive active material will expand and this will result in the extrusion force to the anode increasing. However, the use of the aforementioned high-elastic graphite will make the anode have strong restorative ability after withstanding high pressure, so that the contact surface between the anode material remains intact, to avoid damage to the interface of the material and the phenomenon of detachment caused by the expansion, which will improve the battery cycle performance without loss of energy density. However, the foregoing explanation is provided for the purpose of facilitating the understanding of the principles of the present application by those skilled in the art and should not be construed as a limitation on application. This application does not exclude the possibility that other principles may be established with the development of technology.

The inventors have further found that the higher the graphite degree of graphite, the greater the battery capacity, but too high a graphitization degree will lead to the reduction of the distance between the graphite layers, and the change of volume caused by lithium ion deintercalation during charging and discharging will be excellent, which will affect the stability of the SEI layer. If the degree of graphitization is too low, the crystallinity of the graphite is low and the network defects will be greater, so side effects are likely to occur in the cycling process that will lead to capacity attenuation. Through a large number of experiments, the inventors found that the degree of graphitization of 94% to 98% is correct, preferably 94% to 96%.

The inventors have further found that when the D50 of the graphite is more than 18 pm, it will make the number of the stacking layers of the particles less, and it is difficult to form an elastic structure. When the D50 is less than 6 pm, the bonding force between the materials is too weak, so the adhesion to the electrode plate will be poor, during the cycle peeling phenomenon is likely to occur and will lead to attenuation Of capacity. Therefore, the graphite should have a D50 average particle size of 6 pm to 18 pm, preferably 6 pm to 12 pm.

[0020] In order to further improve the speed performance, the graphite surface may also have a coating layer. The coating layer is normally an amorphous carbon, for example at least one selected from the group consisting of carbon black, coke, soft carbon, and hard carbon. The amorphous carbon content with respect to the total weight of the electrode material is generally 2% to 13%, preferably 5 to 10%. In some embodiments, amorphous carbon is obtained by carbonization (at high temperature) of at least one material selected from the group consisting of polyvinyl butyral, bitumen, furfural resin, epoxy resin, or phenolic resin.

[0021] The lithium ion secondary battery comprising the aforementioned specific positive material and specific negative material can be prepared by a method known in the field, such as the following:

1. Cathode preparation

[0022] In general, the positive active material, the conductor and the binder are mixed in a certain proportion by weight, then the solvent is added and the mixture is stirred under the action of a vacuum stirrer in a uniform transparent state to obtain a suspension of positive material; cover the positive current collector with the positive material slurry; then dry it and slit it to obtain a cathode.

[0023] The positive active material used in the present application is LixNiaCobMcO2, where M is at least one selected from the group consisting of Mn and Al, 0.95 <x <1.2, 0 <a <1.0 < b <1, 0 <c <1 and a bc = 1. When M is Mn, the material's formula is abbreviated as NCM; when M is Al, the material's formula is abbreviated as AQL. Materials can be purchased from suppliers.

Specifically, the positive active material can be at least one selected from the group consisting of LiNi0.33Co0.33Mn0.33O2, LiNi0.5Co0.2Mn0.3O2, LiNi0.5Co0.25Mn0.25O2, LiNi0.6Co0.2Mn0, 2O2, LiNi0.8Co0.1Mn0.1O2, LiNi0.85Co0.1Mn0.05O2 and LiNi0.8Co0.15Al0.05O2.

[0025] In a preferred embodiment of the present application, the content of the positive active material is 94% to 98% by weight based on the total weight of the layer of positive material.

[0026] In some embodiments of the present application, the positive active material can also be doped with at least one element selected from the group consisting of Al, Zr, Ti, B, Mg, V, Cr, F, in order to further improve battery performance.

[0027] In some embodiments of the present application, forming a coating layer on the exterior of the crystal of the positive active material can further improve the performance of the battery, and the coating layer can contain, for example, at least one of the elements Al, Zr, Ti and B.

2. Preparation of anode

1) prepare negative material

In the present application, a graphite having a degree of graphitization of 94% to 98% and an average D50 particle size of 6 gm to 18 gm is used as the negative active material. In the present application, "graphite" has the meaning well understood by those skilled in the art, and it is a carbon material suitable as a battery negative material that is primarily in the form of a sheet of graphite on the inside. The graphite can be natural graphite, artificial graphite, or a mixture thereof. The graphite used in the present application having a degree of graphitization of 94% to 98% and a D50 average particle size of 6 gm to 18 gm can be prepared, for example, by the following method:

(A) grinding the calcined petroleum needle coke or calcined coal needle coke to obtain raw materials with an average particle size of 5-20 gm;

(B) subjecting the raw material obtained in step (A) to a shaping treatment and then subjecting it to a sorting treatment to adjust the particle size distribution of the raw material (preferably large particles having a particle size greater than D90 and small particles having a particle size smaller than D10 are removed);

(C) sieving the raw material obtained in step (B) and then subjecting it to high temperature graphitization, for example, in an Acheson graphitization oven at a temperature of, for example, 2800 ° C to 3250 ° C (preferably 2850 ° C to 3200 ° C);

(D) sieve and demagnetize the material obtained in step (C), to obtain the desired negative material.

The shaping treatment in step (B) is a conventional treatment method in the process of preparing artificial graphite, which is well known to those skilled in the art and can be carried out using any shaping machine or of another forming device commonly used in the art. The sorting treatment in step (B) can be carried out using a sorting screen (sieving method), a gravity sorter, a centrifugal separator or the like. Optionally, after step (C), the Coating carbonization step can be carried out before step (D), that is, the product obtained in step (C) is mixed with at least one material selected from the group consisting of polyvinyl butyral, bitumen, furfural. resin, epoxy resin or phenolic resin and subjected to high temperature carbonization treatment. The temperature of the carbonization treatment is, for example, 900-1500 ° C, for example 1000-1400 ° C or 1100-1300 ° C.

[0030] Alternatively, the present application can also use a natural graphite or commercially available graphite having a degree of graphitization of 94% to 98% and a mean particle size D50 of 6 pm to 18 pm.

The degree of graphitization of graphite can be determined by methods known in the field, for example by X-ray diffractometer (reference, for example, Qian Chongliang et al., "Graphitization Measurement of Carbon Material by X-ray Diffraction" , Journal of Central South University of Technology, Vol. 32, No. 3, June 2001).

[0032] The mean D50 particle size of graphite can be conveniently determined using a laser particle size analyzer (eg Malvern Master Size 2000).

2) Anode mounting

[0033] In general, the negative active material, the thickener, the binder are mixed in a certain proportion by weight; then the solvent is added to obtain the suspension of the negative electrode; then cover the negative current collector with the negative electrode suspension; then dry it and cut it to get an anode.

In a preferred embodiment of the present application, the content of the negative active material is 94% to 98% by weight based on the total weight of the negative material layer.

3. Electrolyte preparation

As a non-aqueous electrolyte, a lithium salt solution dissolved in an organic solvent is generally used. The lithium salts are, for example, inorganic lithium salts such as LiClÜ4, LiPF6, LiBF4, LiAsb6, LiSbF6; or organic lithium salts such as LiCF3SÜ3, LiCF3CÜ2, Li2C2F4 (SÜ3) 2, LiN (CF3SÜ2) 2, LiC (CF3SO2) 3, LiCnF2n + 1SÜ3 (n> 2). The organic solvent used in the non-aqueous electrolyte is, for example, a cyclic carbonate such as ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate; or a chain carbonate such as dimethylcarbonate, diethylcarbonate, and methylethylcarbonate; or a cyclic ester, such as methyl propionate, or a chain ester, such as Y-butyrolactone; or a chain ether such as dimethoxyethane, diethyl ether, diethylene glycol dimethyl ether, and triethylene glycol dimethyl ether; or a cyclic ether such as tetrahydrofuran and 2-methyltetrahydrofuran; or nitriles such as acetonitrile and propionitrile; or a mixture of these solvents.

[0036] For example, ethylene carbonate (CE), methyl-ethyl-carbonate (MEC) and diethyl carbonate (CED) were mixed according to a certain volume ratio, and then the sufficiently dry lithium salt LipF6 was dissolved in a mixed organic solvent to prepare an electrolyte.

4. Separator

[0037] There is no special requirement for the spacer. In particular, the separator can be selected from a polyethylene film, a polypropylene film, a polyvinylidene fluoride film, and a multilayer composite film thereof, depending on the actual requirements.

5. Battery preparation

[0038] Put the spacer between the cathode and anode, then winding, inserting gelatin roll, injection of electrolyte, etc., to get the lithium ion battery.

The advantageous effects of the present application will be further described below with reference to the following examples.

1. Preparation of mounting material and battery

[0040]

(1) The preparation of a cathode: LiNi0.6Co0.2Mn0.2O2, SuperP (conductive agent), PVDF (binder) were mixed in a mass ratio of 97: 1: 2, then a solvent was added. The mixture was stirred in a vacuum mixer in a uniform and transparent system, to obtain the suspension of the positive electrode. The suspension of the positive electrode was uniformly coated on the aluminum foil of the positive current collector; then the aluminum foil was dried at room temperature and then transferred to the oven to dry, and then the cathode was obtained by cold pressing and cutting.

(2) Preparation of an anode: samples of artificial graphite anode active material were taken and measured the particle size of the sample using Malvern Master Size 2000 laser particle size analysis, and the degree of graphitization of the sample was measured using X-ray diffractometer. The test results can be found in Table 1. The artificial graphite negative active material, sodium carboxymethylcellulose (thickener) and SBR (styrene-butadiene rubber binder) were mixed in a mass ratio of 97: 1.2: 1.8 and deionized water was added, then, under the action of a vacuum stirrer, a negative electrode suspension was obtained. The suspension of the negative electrode was uniformly coated on the copper foil of the negative current collector; the copper foil was dried at room temperature and then transferred to the oven to dry, and then the anode was obtained by cold pressing and cutting.

(3) Preparation of an electrolyte: ethylene carbonate (CE), methyl ethyl carbonate (MEC) and diethyl carbonate (DEC) were mixed in a volume ratio of 3: 6: 1, followed by dissolving the salt of Completely dry LipF6 lithium in a mixed organic solvent at a concentration of 1 mol / L to prepare an electrolyte.

(4) Separator: 12 micron PP / PE composite insulation film was used.

(5) The preparation of the full battery: the cathode, the separator, the anode were stacked in order, so that the separator was segregated between the cathode and the anode, and then the battery was wound to obtain a bare cell; the naked cells were placed in the outer wrapper of the packaging. The prepared electrolyte was poured into dry bare cells, and the lithium ion battery was obtained by vacuum packing, standing, chemical treatment, shaping and the like.

2. Cycle performance:

[0041] At 25 ° C, the battery was first charged and discharged as follows: constant current charging and constant voltage charging with a constant current of 1C to the upper voltage limit of 4.2V, then constant current discharging with a constant current of 1C to the final voltage of 2.8V, recording the discharge capacity of the first cycle. The charge / discharge cycles were performed in this manner.

[0042] Cycle capacity retention rate = (discharge capacity of the n ° cycle / discharge capacity in the first cycle) x 100

Examples 2-12 (Example 5 is not according to the invention)

[0043] Example 1 was repeated using different positive active materials and negative active materials. The battery performance data and material parameters are summarized in Table 1.

Comparative Examples 1-4

[0044] Example 1 was repeated using different positive active materials and negative active materials. The battery performance data and material parameters are summarized in Table 1.

[0045] Test result analysis:

1. As can be seen from the analysis of Examples 1-4 and Comparative Examples 1-2:

When the D50 of the negative material was not within the scope of the present application, the cycle performance of the battery was markedly reduced. In Examples 1-4, it was found that when the degree of graphitization was constant, the battery cycle performance gradually decreased with increasing average particle size of the material, preferably in the range of 6-12 pm.

2. As can be seen from the analysis of Examples 2, 5-7 and Comparative Examples 3-4:

When the degree of graphitization of the negative material was not within the scope of the present application, the battery cycle performance was markedly deteriorated. In Examples 2 and 5-7, it can be seen that when the mean particle size of the material was constant, the battery cycle performance gradually decreased with increasing degree of graphitization, preferably in the range of 94 to 96%. %.

3. As can be seen from the analysis of Examples 2 and 11-12, when the positive material was doped and / or coated, the battery cycle performance can be further improved.

[0046] It will be apparent to those skilled in the art that the present application can be modified and varied in accordance with the above teachings. Accordingly, the present application is not limited to the specific embodiments described above, and modifications and variations of the present application are intended to be included within the scope of the claims of the present application. Furthermore, although some specific terminology is used in this specification, these terms are for the convenience of illustration only and are not intended to limit the present application in any way.

Claims (12)

1. A lithium ion secondary battery comprising a cathode, an anode, a separator and an electrolyte, wherein the anode comprises a negative current collector and a negative material layer, and the negative material layer comprises graphite having a degree of graphitization of 94% to 98% and an average particle size D50 of 6 pm to 18 pm as negative active material. 2. Lithium-ion secondary battery according to claim 1, wherein the degree of graphitization of the negative active material is 94% to 96%. 3. Lithium ion secondary battery according to claim 1, wherein the graphite is selected from natural graphite, artificial graphite or a mixture thereof. The lithium ion secondary battery according to claim 1, wherein the cathode comprises a positive current collector and a layer of positive material, and the layer of positive material comprises a positive active material with the formula LixNiaCobMcO2, M is at least one selected from the group consisting of Mn and Al, 0.95 <x <1.2, 0 <a <1, 0 <b <1.0 <c <1 and bc = 1. 5. The lithium ion secondary battery according to claim 4, wherein the positive active material is at least one selected from the group consisting of LiNi0.33Co0.33Mn0.33O2, LiNi0.5Co0.2Mn0.3O2, LiNi0.5Co0.25Mn0 , 25O2, LiNi0.6Co0.2Mn0.2O2, LiNi0.8Co0.1Mn0.1O2, LiNi0.85Co0.1Mn0.05O2 and LiNi0.8Co0.15Al0.05O2. The lithium ion secondary battery according to any one of claims 4-5, wherein the positive active material has a doping element selected from at least one of Al, Zr, Ti, B, Mg, V, Cr and F. The lithium ion secondary battery according to any one of claims 4-6, wherein the positive active material has a coating layer and the coating layer contains at least one element selected from Al, Zr, Ti, and B. The lithium ion secondary battery according to any of claims 1 to 3, wherein the negative active material has a coating layer and the coating layer comprises amorphous carbon. The lithium ion secondary battery according to claim 8, characterized in that the amorphous carbon is obtained by carbonization of at least one material selected from the group consisting of polyvinyl butyral, bitumen, furfural resin, epoxy resin or phenolic resin. A lithium ion secondary battery according to claim 8, wherein the amorphous carbon content is 2% to 13%, preferably 5% to 10%, based on the total weight of the negative active material. The lithium ion secondary battery according to any of claims 1-3, wherein the content of the negative active material is 92% to 98%, based on the total weight of the negative material layer. 12. The lithium ion secondary battery according to any of claims 4-7, wherein the content of the positive active material is 92% to 98%, based on the total weight of the layer of positive material.